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Author Topic:   Geology- working up from basic principles.
The Matt
Member (Idle past 5541 days)
Posts: 99
From: U.K.
Joined: 06-07-2007


Message 16 of 156 (412774)
07-26-2007 8:45 AM


I've also mentioned that igneous material (formed from molten rock) can be intrusive or extrusive. Intrusive means that it cooled and turned to rock within the ground, and extrusive means that it set on the surface. So if we see igneous material among sediments, how would we tell if it is intrusive or extrusive? This will be important for radiometric dating purposes among other things.
Take a look at this diagram:
The grey bits represent igneous material, and the brown is sedimentary rock. Firstly, the principle of cross cutting relationships tells us that if something cuts across layers it must have formed after the original layer was in place. From this we can say that 'discordant' features such as the near vertical one at the right are intrusive. These are known as dykes.
Igneous intrusions that are 'concordant', i.e. are squeezed between layers are known as sills, and are harder to tell apart from lavas, but we can do this by looking for:
-Continuity. Sills can often jump between layers as shown in the lower left.
-Baked margins. Hot lava/magma thermally alters material surrounding it. On lava flows it will only alter the lower surface as there was nothing above it to alter when it was cooling. Intrusive bodies however will have altered rock both above and below. These are the dark brown sections.
-Chilled margins. These are more tricky, but generally only occur in intrusive bodies with finer crystals towards the edge where cooling occurs faster. These are represented by the dark grey. Link
-Eroded fragments. If we find eroded fragments of an igneous layer in the layer above, it must have been exposed at the surface. It should be determined that we are not looking at a major unconformity before we take this as evidence, as erosion could expose a sill and then fragment it.
If we apply these to the diagram we can easily see that the lower rocks are intrusive and the upper is extrusive.
Again, there are more methods than this but I've covered enough for now.

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 Message 17 by jar, posted 07-26-2007 9:39 AM The Matt has replied

  
jar
Member (Idle past 394 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 17 of 156 (412782)
07-26-2007 9:39 AM
Reply to: Message 16 by The Matt
07-26-2007 8:45 AM


Intrusive-extrusive?
If we apply these to the diagram we can easily see that the lower rocks are intrusive and the upper is extrusive.
Can you expand on that for those of us that are old and slow?
What do the terms mean and exactly what in the diagram should we be looking at to determine which is which?
Edited by jar, : close quote

Aslan is not a Tame Lion

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The Matt
Member (Idle past 5541 days)
Posts: 99
From: U.K.
Joined: 06-07-2007


Message 18 of 156 (412790)
07-26-2007 10:35 AM
Reply to: Message 17 by jar
07-26-2007 9:39 AM


Re: Intrusive-extrusive?
Sure. Intrusive means it solidified underground in cavities full of magma*. Extrusive means formed at the surface as old lava* flows or volcanic ash deposits which have erupted on to the surface from volcanic vents.
*Magma is the term used to describe molten rock that has not yet erupted, and lava is that which has.
As for the diagram- I have indicated baked margins with the darker brown colour. The igneous rock at the top has not altered the rock above it because it was not there at the time the lava was erupted- it formed on the surface. Likewise the processes that could produce the weathered fragments on the upper igneous rock only occur at the surface.

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The Matt
Member (Idle past 5541 days)
Posts: 99
From: U.K.
Joined: 06-07-2007


Message 19 of 156 (413349)
07-30-2007 12:50 PM


One more principle then:
Lateral continuity
quote:
strata originally extended in all directions until they thinned to zero or terminated against the edges of their original basin of deposition.
source
Therefore it is not unreasonable to assume that if we see similar strata on opposite sides of a valley, they were once continuous and formed contemporaneously. We may also deduce that if we see a sudden truncation in strata then we are probably looking at a fault.
Corellating strata over large distances is more tricky. If we cannot do this by superposition or by following the strata in boreholes then we could do this by biostratigraphy which I shall discuss shortly.

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 Message 20 by bdfoster, posted 07-30-2007 5:12 PM The Matt has replied

  
bdfoster
Member (Idle past 4878 days)
Posts: 60
From: Riverside, CA
Joined: 05-09-2007


Message 20 of 156 (413389)
07-30-2007 5:12 PM
Reply to: Message 19 by The Matt
07-30-2007 12:50 PM


OK. But lateral continuity is variable and often not very good at all. Continuity in alluvim is very bad and correlation is sometimes impossible between closely spaced borings. Of coarse the basin of deposition in this case may be a only 3 foot wide creek. Terrestrial deposits in general are commonly lense shaped and not very estensive laterally. Even regionally extensive formations are composed of individual beds that are not nearly as extensive. Formations like this are not time horizons and may be different a age at different locations.

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The Matt
Member (Idle past 5541 days)
Posts: 99
From: U.K.
Joined: 06-07-2007


Message 21 of 156 (413402)
07-30-2007 7:29 PM
Reply to: Message 20 by bdfoster
07-30-2007 5:12 PM


Right you are. There are exceptions to this. Would you agree that this is a fair generalisation though, particularly for marine environments? Obviously we are not to take this principle to be a gospel truth.
Working with the example of a small river valley you use- even if we cannot correlate individual beds well, if we saw river terraces of equal height and composition on either side of the valley it would be a fairly good guess that these were once continuous in that they were part of the same floodplain, do you think?

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bdfoster
Member (Idle past 4878 days)
Posts: 60
From: Riverside, CA
Joined: 05-09-2007


Message 22 of 156 (413538)
07-31-2007 10:50 AM
Reply to: Message 21 by The Matt
07-30-2007 7:29 PM


Yes indeed, a fair generalization. Although some of the principles you mentioned are more than just that. Superposition is refered to as a "Law" by some, and I wouldn't argue too strongly against that. It may not have the widespread acceptance of laws of physics, but there are no exceptions to superposition.

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Minnemooseus
Member
Posts: 3941
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
Member Rating: 10.0


Message 23 of 156 (418399)
08-27-2007 11:36 PM


Good page from another topic - Relative Dating Lab page
Vashgun brought up the page here
http://gpc.edu/.../geology/historical_lab/relativedating.htm
Seems like a nice summary of basic geology.
Moose
Edited by Minnemooseus, : Change ID.

Replies to this message:
 Message 24 by The Matt, posted 08-28-2007 4:24 AM Minnemooseus has replied

  
The Matt
Member (Idle past 5541 days)
Posts: 99
From: U.K.
Joined: 06-07-2007


Message 24 of 156 (418433)
08-28-2007 4:24 AM
Reply to: Message 23 by Minnemooseus
08-27-2007 11:36 PM


The Principle of Faunal Succession
Great link, Thanks.
Sorry for the long silence here- I've not had a stable internet connection for a while, but all is sorted now.
Next: The Principle of Faunal Succession
(see Vashgun/Minnie's link)
Fossils are not randomly distributed throughout the rocks of the earth, but rather occur very well sorted in well observed predictable zones. We do not find trilobites after the Paleozoic and we do not find any mammals or birds until the Mezosoic for example. Link. Furthermore species within these broad groups occur in a very well observed and predictable order.
A common misconception here is that the principle uses an assumption of evolution, which isn't true. It was developed in the late 1700s prior to Darwin's birth.
Biostratigraphy is a technique used to make detailed correlations between rocks in different areas using detailed faunal succession. Using the principle of superposition, geologists work out what order the fossils occur in and how extensive/limited their range (for biostratigraphy those with a short time range and large geographic range are most useful). The next step is to try to find the fossils in an area of rocks of similar age. In many cases we can find the same fossils in the same order in rocks separated by huge distances, for example many carboniferous zone fossils used in England can be found to some degree in Eastern Europe, Russia, Asia and the US*. Here is a pretty good page on biostratigraphy.
*No web reference I'm afraid. From:
Ramsbottom (1977) Major cycles of transgression and regression (mesothems) in the Namurian. Proceedings of the Yorkshire Geological Society, Vol 41, pp. 261-291
Edited by Adminnemooseus, : Changed subtitle from "Re: Good page from another topic - Relative Dating Lab page" to "The Principle of Faunal Succession".

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Minnemooseus
Member
Posts: 3941
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
Member Rating: 10.0


Message 25 of 156 (418438)
08-28-2007 6:03 AM
Reply to: Message 24 by The Matt
08-28-2007 4:24 AM


Re: The Principle of Faunal Succession
This all is probably the place to inject the "The rocks date the fossils and the fossils date the rocks" circular argument argument.
My explanation:
In the beginning, the rocks dated the fossils. It was then discovered that certain fossils (index fossils) were characteristic(s) of rocks of a certain age. Thus, if you found a rock unit of otherwise unknown age that contained a certain index fossil, you could date the rock unit by that fossil.
Thus "The rocks date the fossils and the fossils date the rocks" is essentially true, but not circular. Perhaps it could be better stated as "The rocks date the fossils and then the fossils can be used to date other rocks".
Moose

Professor, geology, Whatsamatta U
Evolution - Changes in the environment, caused by the interactions of the components of the environment.
"Do not meddle in the affairs of cats, for they are subtle and will piss on your computer." - Bruce Graham
"The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness." - John Kenneth Galbraith
"Nixon was a professional politician, and I despised everything he stood for ” but if he were running for president this year against the evil Bush-Cheney gang, I would happily vote for him." - Hunter S. Thompson
"I know a little about a lot of things, and a lot about a few things, but I'm highly ignorant about everything." - Moose

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 26 of 156 (418527)
08-28-2007 7:49 PM


Could we use some of this material on the SkepticWiki? We are unfortunately rather short of geologists.
We should of course credit "The Matt, EvC Forums", unless you would prefer your real name to appear, or unless you would like to register and submit articles there yourself.
Cheers.
Edited by Dr Adequate, : No reason given.

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bdfoster
Member (Idle past 4878 days)
Posts: 60
From: Riverside, CA
Joined: 05-09-2007


Message 27 of 156 (418638)
08-29-2007 2:59 PM
Reply to: Message 25 by Minnemooseus
08-28-2007 6:03 AM


Re: The Principle of Faunal Succession
This all is probably the place to inject the "The rocks date the fossils and the fossils date the rocks" circular argument argument.
The use of fossils to date rocks is an example of a an incredibly valuable and versitile technique employed by virtually all fields of science. Once a correlation between two parameters has been established, the correlarion can be used as a measuring tool. Measurement of a property that is known or easy to measure can then be used to determine the value for a property that is unknown or difficult to measure.
Galileo discovered there was a correlation between the volume of mercury and its temperature. He then used the volume of mercury in a glass tube, which is easy to measure, to determine temperature which is difficult to measure.
Paleontologists take advantage of the correlation that has been established between fossil content and stratigraphic position. Fossil content, which is easy to observe, is used to determine stratigraphic position, whch is sometimes difficult to determine.
The use of fossils to date rocks is no more a case of circular reasoning than using volume change to measure temperature.
Edited by bdfoster, : No reason given.

Brent

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Ihategod
Member (Idle past 6029 days)
Posts: 235
Joined: 08-15-2007


Message 28 of 156 (418723)
08-29-2007 11:42 PM
Reply to: Message 1 by The Matt
07-13-2007 7:50 AM


Preaching to the choir
Dude, no offense, your preaching to the choir. Who will debate this? Who here, amongst the ocean of evolutionists, will speak contrary to your position on basic principles of geology? I must attempt to represent.
The first thing I would like to deal with is the law of superposition. This states that sedimentary layers form in a time progressive sequence with the oldest layers at the bottom and the youngest on the top. To view this simply, picture stacking books one on top of the other- the book you put down first will by at the bottom of the pile, and the book you put down last will be at the top. Can we agree this principle is sound?
no it is not sound. Notice this assumption:
picture stacking books one on top of the other- the book you put down first will by at the bottom of the pile, and the book you put down last will be at the top.
This is assuming perfect environment. What if's abound. Example, your cat got a clever idea and decided to put the top on on the bottom. Or the maid mixed the top two out of spite for her poor employment wage. How is that each layer represents a certain age? More to the point; How is it that this is a law of geology? Wouldn't you have to have a foundational religion..., say, uniformitarianism? For this to even hold water?

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AdminNosy
Administrator
Posts: 4754
From: Vancouver, BC, Canada
Joined: 11-11-2003


Message 29 of 156 (418734)
08-30-2007 12:19 AM
Reply to: Message 28 by Ihategod
08-29-2007 11:42 PM


The need to understand analogies
This is assuming perfect environment. What if's abound. Example, your cat got a clever idea and decided to put the top on on the bottom. Or the maid mixed the top two out of spite for her poor employment wage. How is that each layer represents a certain age? More to the point; How is it that this is a law of geology? Wouldn't you have to have a foundational religion..., say, uniformitarianism? For this to even hold water?
I am concerned that you are flooding too many threads without having enough understanding to be able to read the posts you are responding to.
If this continues you will receive short suspensions.
I suggest you pick a particular, basic topic and stick to that one until you learn a bit about the topic involved. If you wish you may set up a "great debate" thread which can be restricted as to who is allowed to post with you on it. This simplifies your task by reducing the volume of replies and often keeps the focus.
Most of your posts make very little sense. You need to slow down and think through your replies more carefully.

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Minnemooseus
Member
Posts: 3941
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
Member Rating: 10.0


Message 30 of 156 (418744)
08-30-2007 1:56 AM
Reply to: Message 28 by Ihategod
08-29-2007 11:42 PM


Law of superposition (revisited)
picture stacking books one on top of the other- the book you put down first will by at the bottom of the pile, and the book you put down last will be at the top.
...Example, your cat got a clever idea and decided to put the top on on the bottom.
While you actually do raise a valid point that can be covered later (tectonic effects on sedimentary rocks), it is not valid in the context of the law of superposition (LoS).
Essentially, the LoS states that when stacking objects, the bottom object has to be there before the next object up. Thus, when stacking books one at a time, the bottom book must be placed first. Then each successive book can be stacked on top of the previous. Likewise for layers of sediments.
Once the sediments are in place and lithofied (solidified) into rock, there are geological processes to rearrange the bottom to top order, namely faulting and folding. In your analogy, that is what your cat is doing. But that is a discussion for a later time and place.
I hope I don't regret having brought this into this topic.
Moose
Edited by Minnemooseus, : Added the "(revisited)" to the subtitle.

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